The present disclosure is related to subterranean formation operations and, more particularly, monitoring the location of a cement plug during a cementing operation.
Hydrocarbon-producing wells (e.g., vertical, deviated, and horizontal wells in a subterranean formation) are generally drilled using a drilling fluid pumped down a drill string and through a drill bit attached to the end of the drill string. The drilling fluid serves, among other things, to lubricate and cool the cutting surfaces of the drill bit, transport drill cuttings to the surface, control formation pressure, and maintain well stability. After drilling is complete, a casing string may be placed in the wellbore through which hydrocarbons will eventually flow. An annulus is formed between the casing string and the face of the wellbore. A cement slurry is pumped through the casing string and displaces the drilling fluid up through the annulus. The cement slurry hardens in the annulus forming a cement sheath. This operation is termed “primary cementing.” Among other things, the cement sheath may keep fresh water zones from becoming contaminated with produced fluids from within the wellbore. As used herein, the term “fluid” refers to liquid phase fluids and gas phase fluids. The cement sheath may also prevent unstable formations from caving in, thereby reducing the chance of a casing collapse and/or stuck drill pipe. Finally, the cement sheath forms a solid barrier to prevent fluid loss or contamination of production zones.
During the cementing process, a cementing head (also referred to as a cement head) houses and releases one or more cement plugs during a cementing operation. The cement head may be arranged on or otherwise mounted on the topmost joint of the casing string or at a location just above the rig floor. A first cement plug, referred to as a “bottom cement plug” (or “bottom plug”) may be used to prevent or minimize contamination of the cement slurry with drilling fluid contained in a wellbore from the drilling operation. Such contamination could result in suboptimal hydration of the cement slurry, thereby compromising the integrity of the cement sheath, for example. The bottom cement plug may be released from the cement head and precede the cement slurry down the inside of the casing string to help separate the cement slurry from the drilling fluid. The bottom cement plug proceeds down the inside of the casing string until reaching a float collar located at or near the bottom end of the casing string, where it lands or “sits.” Continued pressure from cement pumps open a passageway (e.g., a rupture disk, and the like) through the bottom cement plug, thereby permitting the cement slurry to pass through the bottom cement plug and up through the annulus.
A second cement plug, referred to as a “top cement plug” (or “top plug”) may be released from the cement head as the last of the cement slurry enters the casing string. The top cement plug may be substantially similar to the bottom cement plug in most respects, but is solid rather than having a pressure opened borehole. The top cement plug follows the cement slurry down the inside of the casing string as a displacement fluid (e.g., water, seawater, drilling mud, or the like) is pumped behind the top cement plug. The top cement plug proceeds down the inside of the ceasing string until it reaches the bottom cement plug, where it lands or “sits,” signaling a cement plug operator to cease operation of the cement pumps. As used herein, the term “cement plug” will be used collectively to refer to both the bottom and top cement plugs. Accordingly, the cement slurry is located only in the casing string below the cement plugs and in the annulus and the displacement fluid is located only above the cement plugs inside the casing string. Thereafter, the cement slurry is maintained in the annulus until it is hardened to form a cement sheath, as described above.
Optimal cementing operations depend, at least in part, on the location of the cement plugs within the inside of the casing string. Accurate identification of the location of the bottom cement plug, for example, is important to prevent over- or under-displacement of cement slurry. Over-displacement may result in moving all of the cement slurry into the annulus, thereby resulting in a cement deficiency at the bottom of the casing string. Under-displacement results in cement slurry hardening within the inside of the casing string at undesirable locations which must be removed for later production of the well. Accurate identification of the location of the top plug, for example, is important to signal cessation of cementing pumps. Successful placement of the cement plugs (e.g., “bumping” of the cement plugs), therefore, corresponds to a positive indication that the cement slurry has been optimally placed, thereby allowing the casing string to be tested and pressure-activated hangers or tools to operate, minimizing drill-out time, environmental risks, and other costly expenses.